The Invention is based on a drive unit, in particular for a fan in motor vehicles.
Such drive units are used in motor vehicles as fan motors to air condition or heat the passenger compartment, and to cool the coolant of the internal combustion engine. In the case of air-conditioning or heater fans, it is known, for example, that a step switch can be used to select various fan levels at which the DC motor connected to the vehicle""s 12-volt electrical system is set at different speeds. In the case of air-conditioner fans for the coolant of the internal combustion engine, it is known that the speed of the DC motor driving the fan for the radiator can be adjusted using an electrical control unit as a function of engine management data.
A known control unit for controlling a DC motor (DE 197 56 461 A1) comprises a semiconductor switch developed as MOSFET that is connected in series with the DC motor, and the control electrode of which is acted upon by a pulse width modulated control signal. The DC motor is therefore providedxe2x80x94by means of the semiconductor switchxe2x80x94with a mean voltage, the level of which depends on the modulation depth of the control signal. The speed of the DC motor decreases as the mean voltage decreases. The control signal is generated as a function of a setpoint value-actual value comparison of the power of the DC motor. It is also possible to generate the control signal from a set speed that is specified manually, for example, and eliminate a speed closed control loop.
The drive unit according to the invention has the advantage that a stepwise adjustment of the speed of the motor can be realized in very a cost-effective fashion. Since the semiconductor switch is not operated in cyclical fashion as with the known control unit, but is instead operated in such a fashion that one semiconductor switch at a time is fully connected, and the remaining semiconductor switches are fully blocked, the electromagnetic compatibility (EMC) of the drive unit is not critical. When low-resistance semiconductor switches are used, the only power losses that occur in them are negligibly small, so that large-volume heat sinks can be eliminated. The device for controlling speed is compact and is capable of being integrated in a connector of the drive unit according to an advantageous embodiment of the invention, for example.
According to an advantageous exemplary embodiment of the invention, for n+1 speed levels, the device comprises n series circuits, each having one resistor and one semiconductor switch, as well as a further semiconductor switch connected in series with the motor to the DC power supply. Each of the n series circuits is connected in parallel with the semiconductor switch of the series circuit for the subsequent speed level, and the last series circuit is connected in parallel with the further semiconductor switch. Resistors having identical ratings can be used for speed levels that differ by identical amounts in terms of speed, and the semiconductor switches can be dimensioned differently in accordance with their different current loads in order to make use of cost advantages. When the further semiconductor switch is closed, the maximum speed is set.
According to an alternative exemplary embodiment of the invention, for n+1 speed levels, the device has n series circuits, each comprising one resistor and one semiconductor switch, and one further semiconductor switch connected in series with the motor to the DC power supply, whereby the n series circuits are connected in parallel with each other and with the further semiconductor switch. The resistors are to be rated differently in accordance with the selected speed level. In this case as well, however, the semiconductor switches can be dimensioned differently due to the fact that their current load decreases as the speed decreases. When the further semiconductor switch is closed, the maximum speed is set.